Meyers, B. B., & Cafarelli, E. (2002). Caffeine increases endurance without altering average motor unit firing rate during submaximal fatiguing contractions. Medicine and Science in Sports and Exercise, 34(5), Supplement abstract 1428.

This study assessed if improved endurance performance following caffeine ingestion is the result of altered motor unit firing rate during a fatiguing exercise. Males (N = 6) performed under two conditions on two separate days. One hour before exercise, one condition was to ingest 6 mg/kg of caffeine while the other was a similar amount of flour in a capsule form. The exercise was repeated isometric contractions of the quadriceps at 50% maximum voluntary contraction for 15 seconds, with 2 seconds rest between contractions. Exercising was halted when contraction force fell below 40% maximum voluntary contractions for at least two seconds.

The caffeine condition produced a ~27% increase in time to exhaustion which was significantly more than the placebo condition. Motor unit firing rates were similar between conditions at various stages of the exercise.

Implication. Although caffeine is widely reported as exciting actions within the nervous system, its ergogenic effects for endurance performance are not a result of greater excitation.

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